This invention relates to a method for growing silver halide crystals Silver halide crystals grown by this method can be used in photothermographic systems.
In the formation of photothermographic imaging materials, photosensitive silver halide salts have been combined with the substantially light-insensitive silver salts. The light-insensitive silver salts are generally salts of fatty acids. The photosensitive silver halide forms silver when struck with light, and this silver is catalytic to the reduction of the light-insensitive silver salts. Silver halide generally has heen placed in catalytic proximity to the silver salts of fatty acids by physical admixture of the halide salt and organic salt (e.g., U.S. Pat. No. 3,152,904), mixture of preformed silver halide into the precipitating solution for the organic silver salt (e.g., U.S. Pat. No. 3,839,049) and in situ halidization of the dried or coated silver salt of fatty acids (e.g., U.S. Pat. No. 3,457,075).
All of these various methods of producing photothermographic materials require the same materials to be present in the structure and may use the same addenda for modification of sensitometric properties. The required material, in addition to the silver halide in catalytic proximity to the silver salt of an organic fatty acid, is the reducing agent for silver ions. Organic reducing agents, particularly organic photographic developers such as hydroquinone, methyl gallate, catechol, phenylene diamine, p-aminophenol and phenidone, are useful. The hindered phenol reducing agents are the most preferred. Amongst the addenda which may be used in the diverse photothermographic emulsions produced by these processes are toning agents (e.g., U.S. Pat. Nos. 3,392,020; 3,446,648; 3,667,958), sensitizers and sensitizing dyes (e.g., U.S. Pat. Nos. 3,679,422; 3,666,477; 3,761,279; 3,719,495), color couplers (e.g., U.S. Pat. No. 3,531,286), decolorizable light absorbers (e.g., U.S. Pat. No. 3,745,009), mercury compounds (e.g., U.S. Pat. No. 3,589,903), etc. Although silver halide can be provided to the photothermographic imaging system by in situ halidization of the silver salt of fatty acids, it is preferable to introduce preformed silver halide into the imaging system for the following reasons:
(1) the size and distribution thereof of preformed silver halide grains are subject to better control than the size and distribution of silver halide grains formed in situ;
(2) preformed silver halide grains permit better variation and control of the sensitometric properties of photothermographic systems than do silver halide grains formed in situ;
(3) the light sensitive portion of the photothermographic system can be kept separate from the heat sensitive portion of the system until the time for use, thus allowing the application of heat to promote grain growth;
(4) the light sensitive portion of the photothermographic system can be modified, sensitized, or discarded without affecting the heat sensitive portion of the system.
British patent specification No. 1,362,970 discloses a method of preparing preformed silver halide grains suitable for use in a photosensitive heat-processable photographic material by mixing a liquid composition comprising an ionizable silver salt, a hydrophobic polymer peptizer or a polyethylene glycol peptizer, and a solvent, with a liquid composition comprising an ionizable halide and a solvent until the silver halide is formed. The thus-formed silver halide is thereafter mixed with a light-insensitive reducible metal compound, and a reducing agent therefor, and the resulting composition is coated on a support. The ionizable silver salt, e.g., silver nitrate, is typically dissolved in water. The polymer peptizer, generally polyvinyl butyral, and the solvent, e.g., acetone and toluene, are mixed with the aqueous solution for ionizable silver salt. A water-in-oil dispersion is produced by mixing these components. According to patent specification No. 1,362,970, it is believed that the silver halide grains form at the interface between the water droplets and the surrounding organic medium when ionizable halide and its solvent are added to the water-in-oil dispersion. Regardless of the mechanism of grain growth, the method of patent specification No. 1,362,970 suffers from the shortcoming that controlling the size of the silver halide grain is difficult. Control of grain size is important because grain size and grain size distribution affect film speed.
U.S. Pat. No. 3,871,887 discloses a method of preparing a photosensitive silver halide by mixing lithium bromide, silver trifluoroacetate and poly(vinyl butyral) in acetone under controlled conditions. The resulting, fine-grain silver bromide can then be mixed with an oxidation-reduction image-forming combination to provide a photothermographic material. The method fails to disclose how one could control grain size and grain size distribution. Furthermore, silver trifluoroacetate is relatively unstable when compared with such sources of silver ions as silver nitrate.